Lifting devices for lifting heavy loads are known in the art. One type of lifting device is the hoist or crane, which is capable of lifting loads vertically using cables, chains, ropes or the like. The use of flexible members means that a load cannot be rigidly positioned in three dimensional space, which is a disadvantage in certain applications. The manipulator is another type of lifting device capable of both lifting heavy objects and rigidly positioning objects in three-dimensional space. For example, pneumatically assisted manually operated manipulators have found widespread use. These manipulators typically consist of an arm that extends outwardly in a generally horizontal direction from a mast about which it is permitted to rotate and from which it is also permitted to pivot arcuately in a generally vertical direction in a pneumatically assisted manner via a lift cylinder. The arm may include one or more extension members serially disposed from an end of the arm distal from the mast that are similarly permitted to rotate and/or pivot. This arrangement permits the positioning of the distal end of the arm or extension member at a desired location in three dimensional space. An end effector is an attachment coupled to the distal end of the arm or extension member and adapted for the manipulation of a desired object or for the conduct of a particular task. For example, an end effector may include clamping means, pincer means, magnetic means or the like that are shaped and/or sized for the securement of a desired object to be positioned.
There are generally at least two types of manipulators. A conventional manipulator uses push-button controls connected directly with the lift cylinder to manually raise or lower the load by increasing or decreasing cylinder pressure. This type of manipulator includes no regulators or other pressure control system and functions merely by controlling the lift cylinder directly. Another type of manipulator is the load balancing manipulator. In a load balancing manipulator, a regulator in fluid communication with the lift cylinder is adjusted to a pre-determined pressure selected so that the weight of the object being manipulated is just balanced by the cylinder. In this manner, the object becomes weightless with respect to the operator, who is then able to manually position the object in three dimensional space. This allows the operator to quickly move and accurately position a work piece at a desired location in order to perform a certain task.
One problem with prior art load balancing manipulators is that the pressure of the cylinder must be pre-selected according to the weight of the load being balanced. This means that only loads of a single pre-determined weight may be moved using the manipulator. Normally, two regulators are provided; a first regulator for balancing the “no load” weight of the manipulator itself at a first pressure and a second regulator for balancing the “with load” weight of the manipulator and the load being carried at a second pressure greater than the first pressure. In certain instances, additional third or fourth regulators may be provided to allow the operator the flexibility to select additional discrete weights. This arrangement of pneumatic components means that the manipulator is fundamentally limited in only being able to lift a limited number of pre-selected weights.
Attempts have been made to make the second regulator dial-adjustable so as to allow an operator to adjust the pneumatic pressure until a particular load is balanced. Although this removes the limitation on pre-selection of the weights, the manipulator must be re-adjusted each time a load of different weight is lifted. This approach still limits the manipulator in terms of the flexibility to quickly lift any desired weight.
Attempts have been made in the prior art to provide automatically self-adjusting load balancing hoists for lifting any attached weight; however, these attempts have not been put into widespread commercial use with manipulators. In addition, fully automatic systems requiring no operator decision making to determine the pressure required to balance any particular load are complicated and expensive. A semi-automatic system that relies upon an operator decision to determine an appropriate load balancing pressure for the load being lifted is less complicated and often provides greater operator confidence than a fully automatic load balancing system.
U.S. Pat. No. 4,500,074 describes a fluid operated hoist having a pilot fluid controlled regulator for setting the load balancing fluid pressure of the hoist. A pilot fluid regulator is carried by a load carrying unit of the hoist. A linkage automatically adjusts the pressure provided by the pilot fluid regulator in response to lifting of the load to thereby adjust the pilot fluid controlled regulator. A manual bypass may also be engaged to actively lift the load. The linkage system adds complexity to the load carrying unit of the hoist and is potentially susceptible to breakdown. This system also uses at least two regulators to achieve load balancing; it would be desirable to reduce the number of regulators in order to reduce system complexity and increase reliability.
U.S. Pat. No. 3,526,388 describes a load balancing hoist having a novel valve mechanism for automatically sensing the pilot pressure necessary to balance a particular load. The novel valve is located on a load carrying member of the hoist. The novel valve introduces cost and complexity to the system and it would be desirable to eliminate the need for the novel valve.
U.S. Pat. No. 3,758,079 describes a load balancing hoist having a pressure sensor in the hoist chamber that is used to automatically determine the pressure required to lift the load. This system uses electronic controls, making it more complicated and expensive to implement and repair, and is susceptible to electronic malfunction.
U.S. Pat. No. 5,613,419 discloses an electronically controlled manipulator capable of lifting a plurality of loads. The manipulator employs electronic load cells to sense a load condition and has a microprocessor that automatically controls a valve in order to apply appropriate cylinder pressure to balance the load. Electronic controls introduce cost, complexity and likelihood of malfunction. It would be desirable to eliminate electronic controls, sensors, etc. in a load balancing manipulator.
U.S. Pat. No. 5,816,132 describes a load sensing pneumatic manipulator that automatically senses the outlet pneumatic pressure needed to maintain the load in a static condition and automatically adjusts this outlet pressure. Control over outlet pressure is less responsive and reliable than control over pressure applied directly to the lifting cylinder. To the knowledge of the inventor, this system has not been implemented commercially.
The devices disclosed in these prior art references are fully automatic, complex and not in widespread commercial use. None of these prior art references discloses a semi-automatic load balancing device capable of lifting any desired weight that is simple to operate and maintain. There is therefore still a need in the art for improved multi-weight balancing devices. Although many of the principles used in load balancing hoists may be applicable to pneumatic manipulators, significant differences can still exist. For example, in many applications it is important to be able to rigidly position a load in three dimensional space, which many load balancing hoists are unable to do. There is therefore a particular need in the art for improved multi-weight balancing pneumatic manipulators.
At present, most manipulators float the load in a balanced state, switching between a no-load balance point and a loaded balance point. The switching between states is normally done abruptly by causing a valve to switch from the no-load air pressure to the load pressure, and vise-versa. This abrupt switching poses the problem that it can sometimes lead to un-natural motion, which is disconcerting to operators. It would be desirable to improve upon this by providing more natural switching between conditions.
In addition, manipulators must be designed to be intrinsically safe in the event of a failure or unforeseen work condition. As a safety feature, there is usually a velocity fuse which restricts air flow if the flow in and out of the lift cylinder if it goes over a certain speed, or equipment similar to the velocity fuse to similarly limit air exhaust speed. Another speed limiting device that is sometimes used is a brake that engages when the manipulator arm is moved too quickly. As an additional safety measure, there may also sometimes be provided a device that bleeds the air in the lift cylinder if the supply pressure fails.
There are normally 3 emergency situations to deal with:    1. the loss of the load, where the load falls out of the jaws of the end effector, and the manipulator arm accelerates upwardly, out of control;    2. the loss of air supply pressure, for example due to an airline break on the manipulator or outside the manipulator, where the manipulator suddenly collapses, out of control; or,    3. an accidental activation of the end effector release mechanism.
In the first situation, the velocity fuse acts to trap the air in the lift cylinder, in a situation where the air should actually be exhausted as quickly as possible. The velocity fuse therefore acts to worsen the danger in a lost load situation.
In the second situation, most existing safety systems sensing the loss of air pressure employ a valve that either locks the air in the lift cylinder or drains it away slowly. They will not generally sense that the pressure is below a safe level; they sense only that the supply has been completely lost. In many situations, the supply drops to an intermediate level that is somewhat less than the minimum necessary to lift the load, but is still a substantial pressure insufficient to cause activation of the lift cylinder locking valve.
In the third situation, the objective generally is to quickly lower the lift cylinder pressure until it is at a level low enough that it is safe to open the jaws. This safe level can be the no-load pressure set to balance the weight of the manipulator when no load is present. When pressed prematurely, while the load is unsupported, the effect is a rapid descent of the manipulator, until the velocity fuse or brake limits the rate of descent. If the release mechanism has been pressed, it is desirable to lower the object as quickly as possible; however, rapid lowering of the object is impeded by the velocity fuse.
One additional problem caused by the use of a velocity fuse or a brake is that both of these devices undesirably limit the speed of movement of the manipulator during normal use. These devices must be set so that they engage quickly in a safety event. One drawback therefore is that these devices can sometimes be triggered when working quickly with the manipulator, as rapid movements of the operator cause air to exhaust at a rate comparable to the trip setting of the velocity fuse. This is disconcerting to operators, since the manipulator locks up and prevents further movement until the fuse is re-set. This can lead to widespread loss of production efficiency, particularly in an assembly line setting. Unlike hoists, which are provided to lift very heavy loads, manipulators are often provided to speed up work by allowing an operator to more naturally and quickly lift loads of a moderate weight; the problem of velocity fuse tripping is therefore more prevalent in manipulators than in hoists.
U.S. Pat. No. 3,791,627 discloses a load balancing hoist having three different bleed valves on the outlet of the main lift cylinder. The first two valves are for lowering the load by relieving air pressure from the lift cylinder. The third valve allows a slower release of air and is engaged as an alternative to the first two valves in the event of loss of air supply pressure. This allows the load to slowly be lowered to the ground. This system works only in the event of air supply failure, rather than lost load condition and is applicable to a hoist, rather than a manipulator.
There is therefore still a need in the art for improved safety devices for use with load balancing pneumatic manipulators, particular in the event of a lost load condition.
Various types of actuators are used to operate pneumatic valves. One type of actuator is the manual push button. Another is a solenoid operated via an electric switch. It is also known in the art to employ a neutrally biased sliding actuator or shuttle that reciprocates between upper and lower positions in order to operate two valves in a mutually exclusive fashion. However, these actuators are not known in combination with a semi-automatic system for balancing any load and can provide more intuitive operation than other actuator systems.
U.S. Pat. No. 3,880,393 discloses a manually operated shuttle valve containing two spring returned three way valves. The springs maintain the shuttle in a neutral position. When an operator lifts on the shuttle, an upper valve is engaged and when the operator pushes downwardly on the shuttle, a lower valve is engaged. These valves are connected to different portions of the pneumatic load balancing circuit.
U.S. Pat. No. 5,269,644 discloses a load balancer having a manual control switch comprising a reciprocating sleeve centrally spring balanced between two inductive proximity switches. Movement of the sleeve upwardly causes the upper switch to engage, and the lower switch to disengage; the opposite also applies. The shuttle is located near an operator controller of the load balancer.
The advantages of this type of actuator in combination with a semi-automatic load balancing pneumatic lifting device have not been previously realized.